October surprise: Kids born in fall least likely to get the flu

BOSTON — October is usually synonymous with ghosts, goblins, and pumpkins, but new research suggests the spookiest time of the year is also a great time to avoid the flu! Scientists at Harvard Medical School and Massachusetts General Hospital explain that kids born in October are both more likely to receive vaccinations against influenza and least likely to get the flu in comparison to other children.

These findings reveal that birth month correlates with both timing of flu vaccinations and the likelihood of a subsequent case of the flu. On a more practical level, the study suggests October is the best time for young children to get a flu shot, in line with current recommendations.

Annual influenza vaccination is especially important for young children, who are historically at a higher risk of both flu and severe infections requiring hospitalization. Health officials generally recommend vaccinations should take place between September and October in order to maximize immunity during peak flu season.

Among young children in the United States, preventive care visits tend to take place during birth months and can usually serve as a convenient time to receive a flu shot. However, large scale studies of the optimal timing of vaccination remain unavailable. So, study authors set out on their own to analyze the optimal timing of influenza vaccination in young kids.

Using health insurance claims data, researchers identified more than 800,000 children between the ages of two and five who received an influenza vaccination between Aug. 1 and Jan. 31 from 2011 to 2018. Then, they analyzed rates of diagnosed influenza among those kids according to birth month.

Even after the research team accounted for a variety of potentially influential factors including age, sex, existing health conditions, healthcare use, and family size, the ensuing results still showed October as the most common month for children to receive a flu shot.

Children born in October also showed the lowest rates of influenza diagnoses. For instance, among children born in August, the average rate of influenza diagnosis during flu seasons was three percent – compared with 2.7 percent for children born in October and 2.9 percent among children born in December.

The research team stresses that this study was only observational in nature — meaning they can’t directly link birth months to flu rates, but there is a clear trend. They also acknowledge the findings only encompass insured children who received medical care. As such, study authors cannot rule out the possibility that other unmeasured factors possibly influenced the results.

However, results remained similar after researchers performed additional analyses aimed at evaluating if the connection between birth month and influenza risk was due to chance, providing greater confidence to the study’s conclusions.

“Our findings suggest that US public health interventions focused on vaccination of young children in October may yield the best protection in typical flu seasons,” the study reads per a media release. “The study’s findings are consistent with current recommendations promoting October vaccination.”

The study is published in The BMJ.